Generally, an LED is advantageous because it has low power consumption and a long lifetime, and also, it can be mounted in limited spaces and is highly resistant to vibration. Recently, single color LEDs, for example, red, blue or green LEDs, and as well, white LEDs have been disclosed. Of these LEDs, the white LED is expected to be increasingly used as it is applied to automotive products and lighting fixtures.
In the LED techniques, white color is mainly realized by the two following methods. In the first method, red, blue and green light emitting diodes are disposed to be adjacent to one another, and colors of light emitted by the light emitting diodes combine to realize white light. However, since the light emitting diodes have different thermal or temporal properties, the hue changes depending on usage conditions. In particular, non-uniform color, such as color irregularity, is exhibited.
As a second method, a phosphor is included in the light emitting diode, and thus, primary light emitted by the light emitting diode is partially combined with secondary light converted into a predetermined wavelength by the phosphor to realize a white color. This method, for example, adopts a blue light emitting diode having a peak emission wavelength between 450 and 490 nm and a YAG phosphor to absorb light emitted by the blue light emitting diode and convert it into almost yellow fluorescent rays, and is disclosed in WO 98/05078 and WO 98/12757.
However, the above second method is disadvantageous because it cannot provide high white emission properties due to the YAG phosphor. That is, the YAG phosphor exhibits high emission efficiency only in the wavelength range lower than a peak emission wavelength of 560 nm. Thus, when such a phosphor is mixed with a blue light emitting diode having a wavelength between 450 and 490 nm, a low color temperature of 6000 to 8000 K and a low color rendering index of 60 to 75 are manifested, thus exhibiting a cool blue-white color.
Therefore, limitations are imposed on applications of the conventional white LEDs to various products. In particular, such an LED cannot be used for lighting fixtures.